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Studies of atmospheric aerosols in large urban areas using PIXE: an overview
Nuclear Instruments and Methods in Physics Research B 109/110 (1996) 439-444
Beam Interactions with Materials & Atoms
ELSEVIER
Studies of atmospheric aerosols in large urban areas using PIXE: an overview Javier Miranda Instituto de Ffsica, Universidad Nacional Autdnoma de Mdxico, Apartado Postal 20-364, Mdxico, D.F. 01000, Mexico
Abstract The uncontrolled growth of some cities around the world has produced an increasing concentration of atmospheric aerosols. They have effects on the air quality in those geographical areas, affecting public health, visibility, and buildings. A better knowledge of the aerosol characteristics is required. A comparison of the sources, sampling procedures, analytical use of particle induced X-ray emission (PIXE), and composition of atmospheric aerosols in several urban areas around the world, having more than four million inhabitants, is presented. The need for standard procedures and analysis, as well as the use of other techniques in addition to PIXE, is pointed out. It is shown how local sources (traffic, industry, soil), geographical characteristics, or external sources, influence the aerosol composition.
1. Introduction Proton induced X-ray emission (PIXE) has been applied to many different fields of knowledge, due to its high sensitivity, multielemental capabilities, and nondestructiveness [1]. One of the first research fields that made use of PIXE as an analytical technique was related to environmental studies, and in particular, atmospheric aerosols. In the 1991 PIXE survey presented by Cahill et al. [2], 30% of the respondant groups had aerosol research as its primary interest, while other 15% dedicated at least part time to this area. Comprehensive reviews on the use of PIXE as a tool for aerosol analysis have been published by Koltay [3] and Maenhaut [4]. These authors point out the possible effects that the aerosols have on public health, visibility, and building damage. The aforementioned consequences may obviously be enhanced in urban areas, where people are more susceptible to them, and most of the anthropogenic pollutant sources are found. Moreover, when these urban areas accommodate a high number of inhabitants, the coincidence of the sources, population and/or geographic conditions may certainly multiply the effects of the pollutants. Aerosols in large urban areas, being one of the many pollutants present there, should also be a problem of great interest for the PIXE community. The aim of this work, thus, is to discuss the aerosol studies carried out in large urban areas (having more than four million inhabitants), comparing the sampling and analytical * Work supported in part by DGAPA-UNAM under contract IN- 100493.
methods employed, trying to contrast the results obtained in those investigations, and pointing out possible future trends in the large urban areas aerosol studies.
2. Urban aerosols The characteristics of the aerosols in an urban area are dissimilar, in general, to those of the airborne particles in rural or remote zones. The reason lies in the different types of emitting sources. In a "typical" urban area, there is a combination of discharges from automobiles, industry, houses, local dust, organic emissions, and others with possible external origins, such as sea-salt or volcanic eruptions. These aerosols may have been produced directly as particles, or as the result of chemical reactions among polluting gases [5]. The chemical and physical properties of the aerosols produced by the various sources are different, resulting in a possibility of origin identification by performing elemental analysis of aerosol samples. Table 1 shows some of the elements found in samples from urban areas, with their frequently associated emitting sources. This is the context in which PIXE had the best influence because, thanks to its sensitivity and multielemental analysis capacity, it provides fast and reliable results. It is also apparent in Table 1 that some of the elements cited there are not detectable by PIXE, but they can be measured with the aid of other techniques, prior to, simultaneously, or after the application of PIXE [1]. A significant feature of urban aerosols (and sometimes aerosols in other areas), is the size distribution. It has been
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Table 1 Sources and elements found in urban aerosols Source
Elements
Soil Smoke Automobile (gasoline) Fuel oil Coal burning Industry, smelters Sea salt Refuse, domestic incineration Sulfate
A1, Si, K, Ca, Ti, Mn, Fe, Sr K C, Br, Pb V, Ni S Cr, Mn, Fe, Cu, Zn, As, Se, Pb Na, CI As, Br, Zn H, N, O, S
known for a time that they present a bimodal distribution, with a minimum at around 2.5 p,m. The coarse, or mechanical, fraction has a mean at about 30 Ixm, and is mostly produced by windblown dust. On the other hand, the fine fraction is mainly anthropogenic, and it may have direct effects on public health, as these particles are deposited deeply into the lungs. Thus, it is meaningful to study these aerosols also from the viewpoint of size distributions.
3. Studies in large urban areas
According to the World Almanac [6], there were 45 urban areas in the world having more than four million inhabitants in 1985, increasing to 52 cities in 1991. Most of them are located in Third World countries, which also have the highest growth rates. It is unnecessary to say that the effects of population expansion on the air quality of these areas, with the subsequent health damage, should equally increase. Moreover, the degradation of the air quality in surrounding areas, or even remote sites, is indeed important, because of contaminant transport, as demonstrated by Winchester et al. in the case of Beijing, China [7]. This fact explains the necessity of air pollution studies in large urban areas, contrasted to the interest in smaller urban areas, which may have proportionally weaker influences. However, the knowledge of air quality in smaller cities must still be recognized as a powerful aid to the understanding of pollution issues in large urban areas. While pollution research in most of these cities is under progress, studies of atmospheric aerosols have been carried out in about 50% of those areas, and PIXE has been used, during the last 15 years, in no more than 12 megacities, that is, only about 25%. Table 2 displays these cities, including the years during which the studies were done, the sampling devices employed in the course of the investigations, and the type of filters used. In a few cases, more than one group used PIXE to characterize the aerosols at different dates in the same urban area. A couple of statements can be made when looking at Table 2. First, there is a wide variety of sampling devices
used during the studies, although the most common is the stacking filter unit (SFU), which allows particle size separation in two sequential filters: one for particles between 15 and 2.5 p,m, and the other with sizes below 2.5 p~m. Also, cascade impactors or similar mechanisms have been used to obtain a finer size resolution. Although the information resulting from the latter sampling methods is very complete, their use can only be limited to short periods, due to the lengthy elemental analysis. Moreover, the different size selection by the samplers prevents direct comparisons of the results to be carried out. This is a problem that cannot be overlooked, as these comparisons are necessary to evaluate the air pollution problems not only in large urban areas, but in any study regarding atmospheric aerosols. On the opposite side, the use of samplers with no particle size information is inconvenient for presenting such comparisons. Also, the most common filter for aerosol collection by far is the polycarbonate Nuclepore. Probably, this popularity is due to its low cost, availability, radiation resistance, and ease of handling. Although not well suited for other analyses, such as proton elastic scattering analysis (PESA) for hydrogen detection, it can be used reliably for PIXE or XRF examinations. In several cases, though, particle adhesion to these filters is limited. Thus, in this aspect most of the studies are compatible. Additionally, the problems of sample analysis in filters not recommended for PIXE (fiber glass), were overcome by dissolving the collected matter and depositing it on Nuclepore filters [28]. Table 3 contains data related to some experimental parameters used for PIXE analysis, including beam type and energy, and the accelerator. Unfortunately, in one of the works these experimental conditions were not described [14]. It is to be noted that many studies were based on only a few accelerators, that is, the same research groups were involved in them: the University of FloridaElemental Analysis Corporation (Beijing, New York City, St. Petersburg, Sao Paulo), the University of Gent (Sao Paulo, Rio de Janeiro), and the University of CaliforniaDavis (Los Angeles, Mexico City). These three laboratories have conducted studies in more than half of the urban areas of interest for the present work. However, continual monitoring has been done in four areas: Beijing, Kyoto, Milan and Sao Paulo. This has been possible because there are local PIXE groups interested in aerosol analysis. As a result, these cities offer better chances for receptor model investigations (see next section). The research groups have used most of the times other complementary techniques, and some tried to compare the results obtained with PIXE and the other methods. This obviously makes the results more comprehensive, as explained above. However, in several instances PIXE analysis was not used to take advantage of its full multielemental capabilities, focusing on only one element (Buenos Aires [10] and Mexico City [19] for Pb, New York city for S [27]). Moreover, in one of the latter cases
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Table 2 Large urban areas, sampling devices and filters for aerosol studies using PIXE City
Year
Sampling device
Type of filter
Ref.
Beijing
1980
Nuclepore, Mylar Mylar, Gelman film Nuclepore Nuclepore
[7,8]
Madrid
1992
Cyclonic
Mexico City
1988-1990 1990-1991
Open impactor SFU
1993
SFU
1977-
Cascade impactors Integral sampler Streaker; Cascade Hi-Vol. SFU
Mylar, Nuclepore Teflon Nuclepore, Mylar, Teflon Millipore cellulose Nuclepore Nuclepore, Teflon Nuclepore, Teflon Nuclepore
[ 13]
1983, 1987-88 1986
Cascade impactor 8-stage Cascade Open impactor SFU; Rotating Stage Cascade Multiday impactor Dichotomous SFU, DRUM
Nuclepore
[26]
Nuclepore; Mylar Fiber glass, Nuclepore Kimfoil, Nuclepore Mylar
[27]
1983-1984 Buenos Aires Kyoto-Osaka
1988 1986-1992
Los Angeles
1973
Milan
1990 New York City
1977
Rio de Janeiro
1990
St. Petersburg
1989
Sao Paulo
1976-1977
Tokyo
1983 1989 1990 1989
[10] the analysis was performed with 50 MeV ~2C and ions, instead of protons.
Total; 2-stage Streaker Cascade impactors SFU SFU Hi-Vol, SFU Mini-SFU 160
4. Results of PIXE studies
The results of the studies of aerosols in large urban areas based on PIXE are as diverse as the cities themselves. The geographical and climatic characteristics, together with the traffic density and industrialization of the region, affect the composition of the aerosols. Although direct comparisons are difficult to establish, Table 4 shows the concentrations measured in atmospheric aerosols for some representative elements and urban areas. Where applicable, fine and coarse fractions have been added, to have a more direct comparison with those studies that did not make any particle size separation. Whenever data from more than one site was published, one of them having concentrations around the average is given in Table 4. In certain works,
Nuclepore Nuclepore Nuclepore Nuclepore
[9] [ 10] [ 11,12]
[14] [ 15] [ 16] [ 17-19] [20,21 ] [22] [23-25]
[28] [29] [30] [31,32] [33,34] [28] [35]
the concentration values in tabular form were not published [16,27,28,30-32]. The elements chosen have normally different origins (see Table 1). Thus, it is interesting to note the contributions of the various sources to the aerosols. The S concentrations (sulfate or coal burning) are in most of the cases between 3 and 5 bLg/m 3, although extreme cases are Milan (of the order of 10 ixg/m3), and St. Petersburg (below 1 I.Lg/m3). Regarding the other elements, there seems to be no clear behavior in their contents. V, as a tracer of fuel oil, in many cases lies below the minimum detectable limits. On the other hand, Fe, normally associated to soil dust, may be highly influenced by the geographical characteristics of the zone. Thus, Mexico City, situated on a normally dry region, has relatively high Fe concentrations, which may also be enhanced by traffic-removed dust. Beijing data was possibly affected by an episodic industrial emission [8], explaining the very high figures for Fe. The origin of Zn, clearly anthropogenic, is definitely dependent upon the
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Table 3 Experimental parameters for PIXE analysis of aerosols in large urban areas City
Year
Ref.
Beam
Accelerator
Beijing
1980
[7.8]
3 MeV H -
Buenos Aires
1983-84 1988
[9] [10]
Kyoto-Osaka
1986-92
[11,12]
NA 50 MeV ~2C, 16O 2.0 MeV H ÷
Los Angeles
1973
[13]
16 MeV a
1983, 1987-88 1986
[14]
NA
Tandetron EAC, florida NA Tandem CNEA Tandem Kyoto Univ. Cyclotron UCD NA
[15]
4.5 MeV H ÷
Madrid
1992
[ 16]
2.55 MeV H ÷
Mexico City
1988-90
[17-19]
4.5 MeV H ÷
1990-91
[20,21]
4.5 MeV H ÷
1993
[22]
2.2 MeV H ÷
1977-
[23-25]
2.8 MeV H ÷
1990
[26]
3.0 MeV H +
New York City
1977
[27]
NA
Rio de Janeiro
1990
[28]
2.4 MeV H +
St. Petersburg
1989
[29]
3 MeV H +
Sao Paulo
1976-77
[30]
3 MeV H ÷
1983
[31,32]
2.4 MeV H +
1989
[33,34]
2.4 MeV H +
1990
[28]
2.4 MeV H ÷
1989
[35]
2.4 MeV H +
Milan
Tokyo
u r b a n zone. Therefore, in M e x i c o City, w h i c h s h o w e d e x t r e m e concentrations [21], m u s t h a v e an industrial source ( b e c a u s e the s a m p l i n g site w a s in the middle o f an i m p o r t a n t industrial zone), while in Beijing it w a s attributed to refuse incineration [9]. Finally, Pb is the characteristic e l e m e n t o f traffic-related aerosols. Here, it is also seen that Pb contents are not related to the n u m b e r o f inhabitants in the city, b u t it s h o u l d rather be influenced by the quality o f the g a s o l i n e c o n s u m e d in that area. T h e b e h a v i o r o f coarse and fine aerosols h a s also been discussed. In m a n y cases, the fine fraction is associated with a n t h r o p o g e n i c sources, while the coarse o n e is m o s t l y natural [22,34]. P r e d o m i n a n t e l e m e n t s in the coarse frac-
Cyclotron UCD Tandem Lund Univ. Cyclotron UCD Cyclotron UCD Van de Graaff U.N.A.M. Tandem Van de Graaff/CISE Van de Graaff Florence Univ. Tandetron EAC, Florida Cyclotron Univ. Gent Tandetron EAC, Florida Tandetron EAC, Florida Cyclotron Univ. Gent Cyclotron Univ. Gent Cyclotron Univ. Gent Van de Graaff Nagoya Univ.
Other techniques
INAA t2C, ~60 ions
XRF PESA FAST Gases LIPM PESA, LIPM PESA, XRF XRF, AAS
AAS XRF
AAS
tion are A1, Si, K, Ca, a n d Fe. O n the other hand, fine aerosols are enriched with S, Zn, a n d Pb. In m a n y cases, the studies were not limited to e l e m e n t a l concentrations. Additionally, receptor m o d e l s or simple multivariate statistical analysis for source identification were applied [9,14,18,22,29,34]. T h e m o s t c o m m o n m e t h od is the absolute principal c o m p o n e n t s analysis ( A P C A ) [36]. This p r o c e d u r e h a s the a d v a n t a g e o f providing relative source contributions to the aerosols, a l t h o u g h the n u m b e r o f s a m p l e s required for reliable results is rather high, especially if a large n u m b e r o f e l e m e n t s are to be associated to the sources. A n o t h e r technique is the c h e m i cal m a s s balance m e t h o d ( C M B ) , w h i c h u s e s r e g r e s s i o n
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Table 4 Concentrations measured in some urban areas for selected elements (p~g/m 3) City
Year
S
V
Fe
Zn
Pb
Beijing Buenos Aires Kyoto Los Angeles
1980 1988 1986-1992 1973 1983 1986 1992 1991 1993 1981 1983-1984 1977 1989 1989 1989
3.99 NA 1.09 4.01 -3.8 2.5 -0.9 4.86 4.91 16.5 10.7 >2.46 0.618 2.90 5.5
<0.093 NA 0.009 0.025 NA NA NA 0.150 0.048 0.30 0.079 NA <0.008 0.031 NA
10.3 NA 0.56 1.01 -0.4 NA -0.6 3.72 2.25 8.8 4.3 NA 0.211 1.96 2.05
0.304 NA 0.088 0.183 -0.1 0.244 -0.04 2.37 0.177 1.1 0.785 NA 0.029 0.238 0.16
0.182 1.5 0.044 1.50 -0.6 0.266 -0.15 1.23 0.10 2.8 1.6 NA <0.012 0.10 -0
Madrid Mexico City Milan New York St. Petersburg S~o Paulo Tokyo
analysis for predicting elemental concentrations [9]. In other cases, cluster analysis (CA) is the method preferred [18], as it only involves correlation coefficients among the variables. The results obtained by CA are normally presented in a graphical form as dendograms. Moreover, the data obtained in Milan have been used to develop a new source profiling technique (source profiles by unique ratios), although it has not been used very widely yet [37]. Using these multivariate statistical techniques, it has been possible to have at least an identification of the pollutant emitting sources, and to know how some geographical, meteorological, or industrial characteristics of an area affect the aerosol composition. For example, in areas adjacent to the sea, marine aerosols have an important
contribution, as is the case of Los Angeles or Silo Paulo [14,34], while sites with dry weather present high concentrations of soil dust (Beijing and Mexico City) [9,21,22]. On the other hand, the effect of traffic on the aerosols is easily distinguished and, as explained before, it depends not only on the number of circulating motor vehicles, but on the gasoline quality, too. Different industrial contributions have also been investigated. Fuel oil and smelters are the main sources of this kind. Other emitters are refuse incinerators, coal burning, and smoke. Table 5 summarizes the multivariate statistical technique and sources identified with them in several urban areas.
5. Conclusions Table 5 Pollutant sources identified in large urban areas using multivariate statistics Urban area
Ref.
Method
Fine
Coarse
Beijinga
[9]
CMB
Los Angeles b
[14]
APCA
Madrid
[16]
APCA
Mexico City
[22]
PCA
St. Petersburg S~o Paulo
[29] [34]
PCA APCA
Soil, coal, limestone, traffic, refuse, oil, marine Traffic, soil, sulfate, marine, fuel oil Traffic dust, traffic gas, refuse Soil, fuel oil. traffic Sulfur Industry, combustion, soil, Cu, Mg
Soil, fuel oil, traffic Dust Soil, industry, combustion, marine
a NO size separation was considered in the multivariate analysis. h No size separation was used in the sampling.
The utilization of PIXE for aerosol characterization in large urban areas was briefly described. The potential of this technique in the subject, improved with other analytical methods, has been known for many years. However, if the number of megacities in the world is considered, it is apparent that PIXE has still an open field. In this regard, it can be said that the airborne particles problem in large cities is widely unknown. While some overpopulated areas present low elemental concentrations, in others with similar geographical characteristics and less populated, the contents are extremely high. Thus, there is not an obvious relationship between city dimensions and air quality. Moreover, the role of PIXE research groups in large urban areas should not be limited to local studies, as most of the megacities in the world do not have a local PIXE group. The impact of these groups will certainly increase if such an attempt is made. On the other hand, the need for standard sampling and analytical procedures is evident. Comparisons of results is very limited because of the variety of samplers, filters, elemental standards or reference materials, etc., used in the
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studies. A n effort in this direction f r o m the PIXE groups i n v o l v e d in aerosol analysis is required. Finally, the application o f receptor m o d e l s or source profiling t e c h n i q u e s s h o u l d b e c o m e a regular p r o c e d u r e for aerosol studies, as they are indeed very helpful for a m o r e c o m p l e t e u n d e r s t a n d i n g o f the problem. This, o f course, w o u l d i n v o l v e a titanic labor, b e c a u s e the n u m b e r o f analysis for reliable results will increase.
References [1] S.A.E. Johansson and J.L. Campbell, PIXE: A Novel Technique for Elemental Analysis (Wiley, Chichester, 1988). [2] T.A. Cahill, J. Miranda and J.R. Morales, Int. J. PIXE 1 (1991) 297. [3] E. Koltay, Int. J. PIXE 1 (1990) 93. [4] W. Maenhaut, Int. J. PIXE 2 (1992) 609. [5] S.E. Manahan, Environmental Chemistry, 6th ed. (Lewis, Boca Raton, 1994). [6] The World Almanac 1994 (Pharos Books, New York, 1994) pp. 774-775. [7] J.W. Winchester, M.X. Wang, L.X. Ren, W.X. Lu, H.C. Hansson, H. Lannesfors, M. Darzi and A.C. Leslie, Nucl. Instr. and Meth. 181 (1981) 391. [8] J.W. Winchester and M.T. Bi, Atm. Environ. 18 (1984) 1399. [9] S. Yin, L. Pingsheng, H. Zhaohui, Z. Ming, Y. Shaojin, Y. Yinan, Q. Qinfang and C. Bingru, Int. J. PIXE 2 (1992) 593. [10] A. Caridi, A.J. Kreiner, JA. Davidson, M. Davidson, M. Debray, D. Jojman and D. Santos, Atm. Environ. 23 (1989) 2855. [11] M. Kasahara, K.-C. Choi and K. Takahashi, Int. J. PIXE 2 (1992) 665. [12] M. Kasahara, K. Yoshida and K. Takahashi, Nucl. Instr. and Meth. B 75 (1993) 240. [13] R.G. Floccbini, T.A. Cahill, D.J. Shadoan, S.J. Lange, R.A. Eldred, EJ. Feeney, G.W. Wolfe, D.C. Simmeroth and J.K. Suder, Environ. Sci. and Technol. 10 (1976) 76. [14] S.H. Ehrman, S.E. Pratsinis and J.R. Young, Atm. Environ. 26B (1992) 473. [15] T.A. Cahill, M. Surovik and I. Wittmeyer, Aerosol Sci. Technol. 12 (1990) 149. [16] A. Climent-Font, E. Swietlicki and A. Revuelta, Nucl. Instr. and Meth. B 85 (1994) 830. [17] F. Aldape, J. Flores M., RN. Diaz, J.R. Morales, T.A. Cahill and L. Saravia, Int. J. PIXE 1 (1991) 355.
[18] F. Aldape, J. Flores M., R.V. Dfaz. J. Miranda, T.A. Cahill and J.R. Morales, Int. J. PIXE 1 (1991) 373. [19] F. Aldape, J. Flores M., R.V. Dfaz and D. Crumpton, Nucl. Instr. and Meth. B 75 (1993) 304. [20] J. Miranda, T.A. Cahill, J.R. Morales, F. Aldape and J. Flores M., Atm6sfera 5 (1992) 95. [21] J. Miranda, T.A. Cahill, J.R. Morales, F. Aldape, J. Flores M. and R.V. Dfaz, Atm. Environ. 28 (1994) 2299. [22] J. Miranda, E. Andrade, A. L6pez-Sufirez, R. Ledesma and T.A. Cahill, Abstracts for the 13th Int. Conf. on the Application of Accelerators in Research and Industry, Denton, TX, USA; 1994, p. 107. [23] E. Caruso, G.M. Braga Marcazzan and E Redaelli, Nucl. Instr. and Meth. 181 (1981) 425. [24] E. Caruso, G.M. Braga Marcazzan and E Redaelli, Nucl. Instr. and Meth. B 3 (1984) 498. [25] G.M. Braga Marcazzan, E. Caruso, E. Cereda, E Redaelli, E Bacci, A.Ventura and G. Lombardo, Nucl. Instr. and Metb. B 22 (1987) 305. [26] G.M. Braga Marcazzan, C. Cavicchioli, F. Lucarelli, E Redaelli, A. Ventura and M. Marchioni, Nucl. Instr. and Meth. B 75 (1993) 230. [27] S.E. Bauman, R. Ferek, E.T. Williams and H.L. Finston, Nucl. Instr. and Meth. 181 (1981) 411. [28] M.H. Tabacniks, C.Q. Orsini and W. Maenhaut, Nucl. Instr. and Meth. B 75 (1993) 262. [29] J.W. Winchester, V.A. Ivanov, M.A. Prokofyev, D.A. Zhukovski, A. Shlikhta, V.F. Zhvalev, A.N. Zhukovski, D.M. Stroganov, J.W. Nelson, S.E. Bauman and L.N. Nakhgaltsev, Nucl. Instr. and Meth. B 49 (1990) 351. [30] L.C.S. Bou6res and C.M.Q. Orsini, Nucl. Instr. and Meth. 181 (1981) 417. [31] C.Q. Orisini, EA. Netto and M.H. Tabacniks, Nucl. Instr. and Meth. B 3 (1984) 462. [32] M.H. Tabacnkis, C. Orsini and E Artaxo, Nucl. Instr. and Metb. B 22 (1987) 315. [33] F. Andrade, C. Orsini and W. Maenhaut, Nucl. Instr. and Metb. B 75 (1993) 308. [34] F. Andrade, C. Orsini and W. Maenhaut, Arm. Environ. 28 (1994) 2307. [35] T. Katoh, S. Amemiya, Y. Tsurita, T. Masuda, E. Koltay and I. Borb61y-Kiss, Nucl. Instr. and Meth. B 75 (1993) 296. [36] W. Maenhaut and J. Cafmeyer, J. Trace Microprobe Techn. 5 (1987) 135. [37] H.J. Annegarn, A. Zucchiatfi. E. Cereda and G.M. Braga Marcazzan, Nucl. Instr. and Meth. B 49 (1990) 372.
Beam Interactions with Materials & Atoms
ELSEVIER
Studies of atmospheric aerosols in large urban areas using PIXE: an overview Javier Miranda Instituto de Ffsica, Universidad Nacional Autdnoma de Mdxico, Apartado Postal 20-364, Mdxico, D.F. 01000, Mexico
Abstract The uncontrolled growth of some cities around the world has produced an increasing concentration of atmospheric aerosols. They have effects on the air quality in those geographical areas, affecting public health, visibility, and buildings. A better knowledge of the aerosol characteristics is required. A comparison of the sources, sampling procedures, analytical use of particle induced X-ray emission (PIXE), and composition of atmospheric aerosols in several urban areas around the world, having more than four million inhabitants, is presented. The need for standard procedures and analysis, as well as the use of other techniques in addition to PIXE, is pointed out. It is shown how local sources (traffic, industry, soil), geographical characteristics, or external sources, influence the aerosol composition.
1. Introduction Proton induced X-ray emission (PIXE) has been applied to many different fields of knowledge, due to its high sensitivity, multielemental capabilities, and nondestructiveness [1]. One of the first research fields that made use of PIXE as an analytical technique was related to environmental studies, and in particular, atmospheric aerosols. In the 1991 PIXE survey presented by Cahill et al. [2], 30% of the respondant groups had aerosol research as its primary interest, while other 15% dedicated at least part time to this area. Comprehensive reviews on the use of PIXE as a tool for aerosol analysis have been published by Koltay [3] and Maenhaut [4]. These authors point out the possible effects that the aerosols have on public health, visibility, and building damage. The aforementioned consequences may obviously be enhanced in urban areas, where people are more susceptible to them, and most of the anthropogenic pollutant sources are found. Moreover, when these urban areas accommodate a high number of inhabitants, the coincidence of the sources, population and/or geographic conditions may certainly multiply the effects of the pollutants. Aerosols in large urban areas, being one of the many pollutants present there, should also be a problem of great interest for the PIXE community. The aim of this work, thus, is to discuss the aerosol studies carried out in large urban areas (having more than four million inhabitants), comparing the sampling and analytical * Work supported in part by DGAPA-UNAM under contract IN- 100493.
methods employed, trying to contrast the results obtained in those investigations, and pointing out possible future trends in the large urban areas aerosol studies.
2. Urban aerosols The characteristics of the aerosols in an urban area are dissimilar, in general, to those of the airborne particles in rural or remote zones. The reason lies in the different types of emitting sources. In a "typical" urban area, there is a combination of discharges from automobiles, industry, houses, local dust, organic emissions, and others with possible external origins, such as sea-salt or volcanic eruptions. These aerosols may have been produced directly as particles, or as the result of chemical reactions among polluting gases [5]. The chemical and physical properties of the aerosols produced by the various sources are different, resulting in a possibility of origin identification by performing elemental analysis of aerosol samples. Table 1 shows some of the elements found in samples from urban areas, with their frequently associated emitting sources. This is the context in which PIXE had the best influence because, thanks to its sensitivity and multielemental analysis capacity, it provides fast and reliable results. It is also apparent in Table 1 that some of the elements cited there are not detectable by PIXE, but they can be measured with the aid of other techniques, prior to, simultaneously, or after the application of PIXE [1]. A significant feature of urban aerosols (and sometimes aerosols in other areas), is the size distribution. It has been
0168-9002/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved S S D 1 016 8 - 5 8 3 X ( 95 ) 0 0 9 4 9 - 3
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Table 1 Sources and elements found in urban aerosols Source
Elements
Soil Smoke Automobile (gasoline) Fuel oil Coal burning Industry, smelters Sea salt Refuse, domestic incineration Sulfate
A1, Si, K, Ca, Ti, Mn, Fe, Sr K C, Br, Pb V, Ni S Cr, Mn, Fe, Cu, Zn, As, Se, Pb Na, CI As, Br, Zn H, N, O, S
known for a time that they present a bimodal distribution, with a minimum at around 2.5 p,m. The coarse, or mechanical, fraction has a mean at about 30 Ixm, and is mostly produced by windblown dust. On the other hand, the fine fraction is mainly anthropogenic, and it may have direct effects on public health, as these particles are deposited deeply into the lungs. Thus, it is meaningful to study these aerosols also from the viewpoint of size distributions.
3. Studies in large urban areas
According to the World Almanac [6], there were 45 urban areas in the world having more than four million inhabitants in 1985, increasing to 52 cities in 1991. Most of them are located in Third World countries, which also have the highest growth rates. It is unnecessary to say that the effects of population expansion on the air quality of these areas, with the subsequent health damage, should equally increase. Moreover, the degradation of the air quality in surrounding areas, or even remote sites, is indeed important, because of contaminant transport, as demonstrated by Winchester et al. in the case of Beijing, China [7]. This fact explains the necessity of air pollution studies in large urban areas, contrasted to the interest in smaller urban areas, which may have proportionally weaker influences. However, the knowledge of air quality in smaller cities must still be recognized as a powerful aid to the understanding of pollution issues in large urban areas. While pollution research in most of these cities is under progress, studies of atmospheric aerosols have been carried out in about 50% of those areas, and PIXE has been used, during the last 15 years, in no more than 12 megacities, that is, only about 25%. Table 2 displays these cities, including the years during which the studies were done, the sampling devices employed in the course of the investigations, and the type of filters used. In a few cases, more than one group used PIXE to characterize the aerosols at different dates in the same urban area. A couple of statements can be made when looking at Table 2. First, there is a wide variety of sampling devices
used during the studies, although the most common is the stacking filter unit (SFU), which allows particle size separation in two sequential filters: one for particles between 15 and 2.5 p,m, and the other with sizes below 2.5 p~m. Also, cascade impactors or similar mechanisms have been used to obtain a finer size resolution. Although the information resulting from the latter sampling methods is very complete, their use can only be limited to short periods, due to the lengthy elemental analysis. Moreover, the different size selection by the samplers prevents direct comparisons of the results to be carried out. This is a problem that cannot be overlooked, as these comparisons are necessary to evaluate the air pollution problems not only in large urban areas, but in any study regarding atmospheric aerosols. On the opposite side, the use of samplers with no particle size information is inconvenient for presenting such comparisons. Also, the most common filter for aerosol collection by far is the polycarbonate Nuclepore. Probably, this popularity is due to its low cost, availability, radiation resistance, and ease of handling. Although not well suited for other analyses, such as proton elastic scattering analysis (PESA) for hydrogen detection, it can be used reliably for PIXE or XRF examinations. In several cases, though, particle adhesion to these filters is limited. Thus, in this aspect most of the studies are compatible. Additionally, the problems of sample analysis in filters not recommended for PIXE (fiber glass), were overcome by dissolving the collected matter and depositing it on Nuclepore filters [28]. Table 3 contains data related to some experimental parameters used for PIXE analysis, including beam type and energy, and the accelerator. Unfortunately, in one of the works these experimental conditions were not described [14]. It is to be noted that many studies were based on only a few accelerators, that is, the same research groups were involved in them: the University of FloridaElemental Analysis Corporation (Beijing, New York City, St. Petersburg, Sao Paulo), the University of Gent (Sao Paulo, Rio de Janeiro), and the University of CaliforniaDavis (Los Angeles, Mexico City). These three laboratories have conducted studies in more than half of the urban areas of interest for the present work. However, continual monitoring has been done in four areas: Beijing, Kyoto, Milan and Sao Paulo. This has been possible because there are local PIXE groups interested in aerosol analysis. As a result, these cities offer better chances for receptor model investigations (see next section). The research groups have used most of the times other complementary techniques, and some tried to compare the results obtained with PIXE and the other methods. This obviously makes the results more comprehensive, as explained above. However, in several instances PIXE analysis was not used to take advantage of its full multielemental capabilities, focusing on only one element (Buenos Aires [10] and Mexico City [19] for Pb, New York city for S [27]). Moreover, in one of the latter cases
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Table 2 Large urban areas, sampling devices and filters for aerosol studies using PIXE City
Year
Sampling device
Type of filter
Ref.
Beijing
1980
Nuclepore, Mylar Mylar, Gelman film Nuclepore Nuclepore
[7,8]
Madrid
1992
Cyclonic
Mexico City
1988-1990 1990-1991
Open impactor SFU
1993
SFU
1977-
Cascade impactors Integral sampler Streaker; Cascade Hi-Vol. SFU
Mylar, Nuclepore Teflon Nuclepore, Mylar, Teflon Millipore cellulose Nuclepore Nuclepore, Teflon Nuclepore, Teflon Nuclepore
[ 13]
1983, 1987-88 1986
Cascade impactor 8-stage Cascade Open impactor SFU; Rotating Stage Cascade Multiday impactor Dichotomous SFU, DRUM
Nuclepore
[26]
Nuclepore; Mylar Fiber glass, Nuclepore Kimfoil, Nuclepore Mylar
[27]
1983-1984 Buenos Aires Kyoto-Osaka
1988 1986-1992
Los Angeles
1973
Milan
1990 New York City
1977
Rio de Janeiro
1990
St. Petersburg
1989
Sao Paulo
1976-1977
Tokyo
1983 1989 1990 1989
[10] the analysis was performed with 50 MeV ~2C and ions, instead of protons.
Total; 2-stage Streaker Cascade impactors SFU SFU Hi-Vol, SFU Mini-SFU 160
4. Results of PIXE studies
The results of the studies of aerosols in large urban areas based on PIXE are as diverse as the cities themselves. The geographical and climatic characteristics, together with the traffic density and industrialization of the region, affect the composition of the aerosols. Although direct comparisons are difficult to establish, Table 4 shows the concentrations measured in atmospheric aerosols for some representative elements and urban areas. Where applicable, fine and coarse fractions have been added, to have a more direct comparison with those studies that did not make any particle size separation. Whenever data from more than one site was published, one of them having concentrations around the average is given in Table 4. In certain works,
Nuclepore Nuclepore Nuclepore Nuclepore
[9] [ 10] [ 11,12]
[14] [ 15] [ 16] [ 17-19] [20,21 ] [22] [23-25]
[28] [29] [30] [31,32] [33,34] [28] [35]
the concentration values in tabular form were not published [16,27,28,30-32]. The elements chosen have normally different origins (see Table 1). Thus, it is interesting to note the contributions of the various sources to the aerosols. The S concentrations (sulfate or coal burning) are in most of the cases between 3 and 5 bLg/m 3, although extreme cases are Milan (of the order of 10 ixg/m3), and St. Petersburg (below 1 I.Lg/m3). Regarding the other elements, there seems to be no clear behavior in their contents. V, as a tracer of fuel oil, in many cases lies below the minimum detectable limits. On the other hand, Fe, normally associated to soil dust, may be highly influenced by the geographical characteristics of the zone. Thus, Mexico City, situated on a normally dry region, has relatively high Fe concentrations, which may also be enhanced by traffic-removed dust. Beijing data was possibly affected by an episodic industrial emission [8], explaining the very high figures for Fe. The origin of Zn, clearly anthropogenic, is definitely dependent upon the
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Table 3 Experimental parameters for PIXE analysis of aerosols in large urban areas City
Year
Ref.
Beam
Accelerator
Beijing
1980
[7.8]
3 MeV H -
Buenos Aires
1983-84 1988
[9] [10]
Kyoto-Osaka
1986-92
[11,12]
NA 50 MeV ~2C, 16O 2.0 MeV H ÷
Los Angeles
1973
[13]
16 MeV a
1983, 1987-88 1986
[14]
NA
Tandetron EAC, florida NA Tandem CNEA Tandem Kyoto Univ. Cyclotron UCD NA
[15]
4.5 MeV H ÷
Madrid
1992
[ 16]
2.55 MeV H ÷
Mexico City
1988-90
[17-19]
4.5 MeV H ÷
1990-91
[20,21]
4.5 MeV H ÷
1993
[22]
2.2 MeV H ÷
1977-
[23-25]
2.8 MeV H ÷
1990
[26]
3.0 MeV H +
New York City
1977
[27]
NA
Rio de Janeiro
1990
[28]
2.4 MeV H +
St. Petersburg
1989
[29]
3 MeV H +
Sao Paulo
1976-77
[30]
3 MeV H ÷
1983
[31,32]
2.4 MeV H +
1989
[33,34]
2.4 MeV H +
1990
[28]
2.4 MeV H ÷
1989
[35]
2.4 MeV H +
Milan
Tokyo
u r b a n zone. Therefore, in M e x i c o City, w h i c h s h o w e d e x t r e m e concentrations [21], m u s t h a v e an industrial source ( b e c a u s e the s a m p l i n g site w a s in the middle o f an i m p o r t a n t industrial zone), while in Beijing it w a s attributed to refuse incineration [9]. Finally, Pb is the characteristic e l e m e n t o f traffic-related aerosols. Here, it is also seen that Pb contents are not related to the n u m b e r o f inhabitants in the city, b u t it s h o u l d rather be influenced by the quality o f the g a s o l i n e c o n s u m e d in that area. T h e b e h a v i o r o f coarse and fine aerosols h a s also been discussed. In m a n y cases, the fine fraction is associated with a n t h r o p o g e n i c sources, while the coarse o n e is m o s t l y natural [22,34]. P r e d o m i n a n t e l e m e n t s in the coarse frac-
Cyclotron UCD Tandem Lund Univ. Cyclotron UCD Cyclotron UCD Van de Graaff U.N.A.M. Tandem Van de Graaff/CISE Van de Graaff Florence Univ. Tandetron EAC, Florida Cyclotron Univ. Gent Tandetron EAC, Florida Tandetron EAC, Florida Cyclotron Univ. Gent Cyclotron Univ. Gent Cyclotron Univ. Gent Van de Graaff Nagoya Univ.
Other techniques
INAA t2C, ~60 ions
XRF PESA FAST Gases LIPM PESA, LIPM PESA, XRF XRF, AAS
AAS XRF
AAS
tion are A1, Si, K, Ca, a n d Fe. O n the other hand, fine aerosols are enriched with S, Zn, a n d Pb. In m a n y cases, the studies were not limited to e l e m e n t a l concentrations. Additionally, receptor m o d e l s or simple multivariate statistical analysis for source identification were applied [9,14,18,22,29,34]. T h e m o s t c o m m o n m e t h od is the absolute principal c o m p o n e n t s analysis ( A P C A ) [36]. This p r o c e d u r e h a s the a d v a n t a g e o f providing relative source contributions to the aerosols, a l t h o u g h the n u m b e r o f s a m p l e s required for reliable results is rather high, especially if a large n u m b e r o f e l e m e n t s are to be associated to the sources. A n o t h e r technique is the c h e m i cal m a s s balance m e t h o d ( C M B ) , w h i c h u s e s r e g r e s s i o n
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Table 4 Concentrations measured in some urban areas for selected elements (p~g/m 3) City
Year
S
V
Fe
Zn
Pb
Beijing Buenos Aires Kyoto Los Angeles
1980 1988 1986-1992 1973 1983 1986 1992 1991 1993 1981 1983-1984 1977 1989 1989 1989
3.99 NA 1.09 4.01 -3.8 2.5 -0.9 4.86 4.91 16.5 10.7 >2.46 0.618 2.90 5.5
<0.093 NA 0.009 0.025 NA NA NA 0.150 0.048 0.30 0.079 NA <0.008 0.031 NA
10.3 NA 0.56 1.01 -0.4 NA -0.6 3.72 2.25 8.8 4.3 NA 0.211 1.96 2.05
0.304 NA 0.088 0.183 -0.1 0.244 -0.04 2.37 0.177 1.1 0.785 NA 0.029 0.238 0.16
0.182 1.5 0.044 1.50 -0.6 0.266 -0.15 1.23 0.10 2.8 1.6 NA <0.012 0.10 -0
Madrid Mexico City Milan New York St. Petersburg S~o Paulo Tokyo
analysis for predicting elemental concentrations [9]. In other cases, cluster analysis (CA) is the method preferred [18], as it only involves correlation coefficients among the variables. The results obtained by CA are normally presented in a graphical form as dendograms. Moreover, the data obtained in Milan have been used to develop a new source profiling technique (source profiles by unique ratios), although it has not been used very widely yet [37]. Using these multivariate statistical techniques, it has been possible to have at least an identification of the pollutant emitting sources, and to know how some geographical, meteorological, or industrial characteristics of an area affect the aerosol composition. For example, in areas adjacent to the sea, marine aerosols have an important
contribution, as is the case of Los Angeles or Silo Paulo [14,34], while sites with dry weather present high concentrations of soil dust (Beijing and Mexico City) [9,21,22]. On the other hand, the effect of traffic on the aerosols is easily distinguished and, as explained before, it depends not only on the number of circulating motor vehicles, but on the gasoline quality, too. Different industrial contributions have also been investigated. Fuel oil and smelters are the main sources of this kind. Other emitters are refuse incinerators, coal burning, and smoke. Table 5 summarizes the multivariate statistical technique and sources identified with them in several urban areas.
5. Conclusions Table 5 Pollutant sources identified in large urban areas using multivariate statistics Urban area
Ref.
Method
Fine
Coarse
Beijinga
[9]
CMB
Los Angeles b
[14]
APCA
Madrid
[16]
APCA
Mexico City
[22]
PCA
St. Petersburg S~o Paulo
[29] [34]
PCA APCA
Soil, coal, limestone, traffic, refuse, oil, marine Traffic, soil, sulfate, marine, fuel oil Traffic dust, traffic gas, refuse Soil, fuel oil. traffic Sulfur Industry, combustion, soil, Cu, Mg
Soil, fuel oil, traffic Dust Soil, industry, combustion, marine
a NO size separation was considered in the multivariate analysis. h No size separation was used in the sampling.
The utilization of PIXE for aerosol characterization in large urban areas was briefly described. The potential of this technique in the subject, improved with other analytical methods, has been known for many years. However, if the number of megacities in the world is considered, it is apparent that PIXE has still an open field. In this regard, it can be said that the airborne particles problem in large cities is widely unknown. While some overpopulated areas present low elemental concentrations, in others with similar geographical characteristics and less populated, the contents are extremely high. Thus, there is not an obvious relationship between city dimensions and air quality. Moreover, the role of PIXE research groups in large urban areas should not be limited to local studies, as most of the megacities in the world do not have a local PIXE group. The impact of these groups will certainly increase if such an attempt is made. On the other hand, the need for standard sampling and analytical procedures is evident. Comparisons of results is very limited because of the variety of samplers, filters, elemental standards or reference materials, etc., used in the
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studies. A n effort in this direction f r o m the PIXE groups i n v o l v e d in aerosol analysis is required. Finally, the application o f receptor m o d e l s or source profiling t e c h n i q u e s s h o u l d b e c o m e a regular p r o c e d u r e for aerosol studies, as they are indeed very helpful for a m o r e c o m p l e t e u n d e r s t a n d i n g o f the problem. This, o f course, w o u l d i n v o l v e a titanic labor, b e c a u s e the n u m b e r o f analysis for reliable results will increase.
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